The congenital adrenal hyperplasias, especially 21-hydroxylase deficiency, are common inherited disorders of steroidogenesis. Correlation of clinical features with molecular genotype has provided a relatively good phenotype/genotype correlation. However, there are patients and families in which the phenotype differs from that predicted for the specific genotype. Investigation of families in which phenotype does not correlate with genotype will provide information about adrenal steroidogenesis. The high prevalence of heterozygosity for 21-hydroxylase deficiency (1/16) suggests the possibility of a heterozygote advantage. Studies are being developed to test this hypothesis.